Semiconductor processing involves multiple steps to produce working integrated circuits on a semiconductor wafer. These steps may include deposition and removal of various materials to form devices and to form the electrical connections between devices. One process that removes material from a wafer is Chemical Mechanical Polishing or Planarization (CMP). CMP generally leaves a planar surface, so that it is particularly suitable for applications where an uneven topology might cause problems, for example where additional layers are to be deposited over a surface formed by previous processing. In particular, where multiple layers of metal interconnection are required in complex integrated circuits, CMP allows successive layers to be formed while maintaining a reasonably flat surface for each successive layer.
CMP generally involves removing material from a wafer by a combination of physical abrasion and chemical action. FIG. 1 shows a cross-section of a wafer 101 that is undergoing a CMP process. The wafer 101 is pressed down into a pad 103 by pressure applied to its upper surface. Pressure may be applied by a plate or pad. In some cases, pressure is applied to the upper surface of the wafer 101 by hydraulic or pneumatic systems. In this way the pressure may be controlled. In some cases, different pressure is provided at different locations on the upper surface of the wafer. Here PE is provided near the edge, while Pc is provided at the center. Different pressures may be provided for a series of concentric zones with separate hydraulic, pneumatic or other systems to separately control pressure. Some relative movement is created between the wafer 101 and the pad 103 by moving one or both of these components. As the wafer 101 moves with respect to the pad 103 its lower surface is eroded. A retaining ring 105 keeps the wafer 101 in position with respect to structures providing pressure Pc and PE to the upper side of the wafer 101. A slurry 107 extends over the pad 103, including the area under the wafer 101. Slurry 107 may be introduced through holes in pad 103. This slurry 107 contains abrasive particles as well as chemical components that may react with material on the wafer. The pad 103 is generally formed of a soft material that deforms under the pressure exerted by a wafer. In general, CMP systems operate so that material is removed from the lower surface of the wafer. Thus, a wafer is generally turned so that the side of the wafer to be processed (generally, the side that contains semiconductor devices and connections) is facing downwards.
In general it is desirable to remove material uniformly across the surface of a wafer. Various parameters may vary across the wafer surface causing nonuniform removal rates. One parameter that may vary across the wafer is the pressure between the pad and the wafer. For example, in FIG. 1, pressure P1 may not be equal to pressure P2. It is generally desirable to know the values of pressure P1 and P2 in order to adjust the CMP process to obtain high uniformity. While the pressures P1 and P2 between the wafer 101 and the pad 103 are affected by the pressures Pc, PE applied to the top surface of the wafer 101, these relationships may not be simple so that measuring Pc and PE may not provide sufficient information for process tuning. Therefore, it is generally desirable to directly measure pressure at points across a wafer surface during processing. CMP processes may also be applied to substrates other than semiconductor wafers, such as flat panel display panels and magnetic heads or still other types of work pieces.